1. JCL didnt say "mass" he said 'inertia'.... the different wheel/tire will have a different rotational inertia.

2. My "3-4 mpg" was a total joke.

3. IMO the difference in 'rolling resistance' is MUCH smaller than all the other components that influence mileage (like 12 square feet of frontal area driving through the air!!) Tirerack did an analysis that showed a whopping 20% increase in rolling resistance would only have a theoretic impact of 3% on mpg. Three percent. While they are in the business of selling wider tires, I thought it was a pretty rational explanation.